Document WO 2011/162903 shows a laser exciting a target material in an appropriate manner to operate as an Extreme Ultra-Violet (EUV) excitation light source. For improving the efficiency of the generation of the EUV light droplets of the target material are struck by a so-called “pre-pulse” and are enlarged, converted into a gas, vaporized or converted into weak plasma. Then the target material prepared by the pre-pulse is converted into plasma by a so-called main-pulse” and emits EUV light. Typically, different lasers are used as light sources for the pre-pulse and for the main-pulse which are introduced into a common amplifier on a common beam path.
The different lasers can also emit light having different wavelengths, wherein, when using several lasers, the laser beams with the different wavelengths are focused on focuses on an axis of the laser beam in different distances due to chromatic aberrations.